The molding of highly porous bodies from thermoplastic materials admixed with an expanding fluid or foaming agent, such as a freon, is well known. The outer surface of such a body may be somewhat less porous than its interior, owing to the pressure to which the injected plastic mass is subjected at the wall of the mold cavity. Even the presence of these finer pores, however, is objectionable in many instances in which the finished article (e.g. an elastic ball) is to be smooth in appearance and/or to the touch. A requirement for absolute impermeability exists in certain cases as, for example, where the molded body is to serve as a receptacle for a fluid to be stored.
Heretofore, articles of this type divided into highly porous and more or less nonporous parts were molded in separate steps, the foamable and the nonfoamable material being successively introduced into the mold cavity from different injection chambers in which these components were subjected to the necessary heat and pressure; see my prior U.S. Pat. No. 3,894,823 and copending application Ser. No. 521,184 filed Nov. 5, 1974 as a continuation of application Ser. No. 286,733 filed Sept. 6, 1972 and now abandoned. Aside from the fact that the provision of such dual injectors is somewhat cumbersome, it has also been found that the layers formed from these separately prepared and injected materials do not always bond firmly to one another even if injection of the foamable material takes place while the previously injected nonfoaming material is still in a semifluid state. This poor adhesion between the adjoining layers is apparently due to the dissimilar histories of their materials.